Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 31410)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -1822,82 +1822,107 @@
  */
 function wp_ajax_update_widget() {
 	global $wp_customize;
 	$wp_customize->widgets->wp_ajax_update_widget();
 }
 
 /**
  * Ajax handler for uploading attachments
  *
  * @since 3.3.0
  */
 function wp_ajax_upload_attachment() {
 	check_ajax_referer( 'media-form' );
 
 	if ( ! current_user_can( 'upload_files' ) ) {
-		wp_send_json_error( array(
-			'message'  => __( "You don't have permission to upload files." ),
-			'filename' => $_FILES['async-upload']['name'],
+		echo wp_json_encode( array(
+			'success' => false,
+			'data'    => array(
+				'message'  => __( "You don't have permission to upload files." ),
+				'filename' => $_FILES['async-upload']['name'],
+			)
 		) );
+
+		wp_die();
 	}
 
 	if ( isset( $_REQUEST['post_id'] ) ) {
 		$post_id = $_REQUEST['post_id'];
 		if ( ! current_user_can( 'edit_post', $post_id ) ) {
-			wp_send_json_error( array(
-				'message'  => __( "You don't have permission to attach files to this post." ),
-				'filename' => $_FILES['async-upload']['name'],
+			echo wp_json_encode( array(
+				'success' => false,
+				'data'    => array(
+					'message'  => __( "You don't have permission to attach files to this post." ),
+					'filename' => $_FILES['async-upload']['name'],
+				)
 			) );
+
+			wp_die();
 		}
 	} else {
 		$post_id = null;
 	}
 
 	$post_data = isset( $_REQUEST['post_data'] ) ? $_REQUEST['post_data'] : array();
 
 	// If the context is custom header or background, make sure the uploaded file is an image.
 	if ( isset( $post_data['context'] ) && in_array( $post_data['context'], array( 'custom-header', 'custom-background' ) ) ) {
 		$wp_filetype = wp_check_filetype_and_ext( $_FILES['async-upload']['tmp_name'], $_FILES['async-upload']['name'], false );
 		if ( ! wp_match_mime_types( 'image', $wp_filetype['type'] ) ) {
-			wp_send_json_error( array(
-				'message'  => __( 'The uploaded file is not a valid image. Please try again.' ),
-				'filename' => $_FILES['async-upload']['name'],
+			echo wp_json_encode( array(
+				'success' => false,
+				'data'    => array(
+					'message'  => __( 'The uploaded file is not a valid image. Please try again.' ),
+					'filename' => $_FILES['async-upload']['name'],
+				)
 			) );
+
+			wp_die();
 		}
 	}
 
 	$attachment_id = media_handle_upload( 'async-upload', $post_id, $post_data );
 
 	if ( is_wp_error( $attachment_id ) ) {
-		wp_send_json_error( array(
-			'message'  => $attachment_id->get_error_message(),
-			'filename' => $_FILES['async-upload']['name'],
+		echo wp_json_encode( array(
+			'success' => false,
+			'data'    => array(
+				'message'  => $attachment_id->get_error_message(),
+				'filename' => $_FILES['async-upload']['name'],
+			)
 		) );
+
+		wp_die();
 	}
 
 	if ( isset( $post_data['context'] ) && isset( $post_data['theme'] ) ) {
 		if ( 'custom-background' === $post_data['context'] )
 			update_post_meta( $attachment_id, '_wp_attachment_is_custom_background', $post_data['theme'] );
 
 		if ( 'custom-header' === $post_data['context'] )
 			update_post_meta( $attachment_id, '_wp_attachment_is_custom_header', $post_data['theme'] );
 	}
 
 	if ( ! $attachment = wp_prepare_attachment_for_js( $attachment_id ) )
 		wp_die();
 
-	wp_send_json_success( $attachment );
+	echo wp_json_encode( array(
+		'success' => true,
+		'data'    => $attachment,
+	) );
+
+	wp_die();
 }
 
 /**
  * Ajax handler for image editing.
  *
  * @since 3.1.0
  */
 function wp_ajax_image_editor() {
 	$attachment_id = intval($_POST['postid']);
 	if ( empty($attachment_id) || !current_user_can('edit_post', $attachment_id) )
 		wp_die( -1 );
 
 	check_ajax_referer( "image_editor-$attachment_id" );
 	include_once( ABSPATH . 'wp-admin/includes/image-edit.php' );